Last Wednesday, Britain's pop aristocracy turned up at London's Earls Court to rehearse for that evening's Brit Awards. As it was only a rehearsal all the top names - Kylie Minogue, Dido, Sting - went through their paces in mufti. Their stage costumes could wait for the evening. Only one performer rehearsed in full kit: Ali G, in gold Lurex jump suit. He has no day wear. He's either full on or he simply doesn't exist. Sacha Baron Cohen, the Jewish Cambridge graduate who performs him, will not let himself and his creation appear anywhere together.

While Ali G speaks, Baron Cohen stays silent, never giving interviews. While everybody knows what the leader of the Staines Massive looks like in his Hilfiger gear and his crusting of gold, only his friends and family would immediately spot Baron Cohen. The separation is total. Admittedly it's not a new trick. Barry Humphries, who created Australia's best loved housewife, does exactly the same, though, unlike Humphries, Baron Cohen won't even talk about his creation. Ali G: a Dame Edna Everage for the twenty-first century.

If anybody had had any doubts about how far Baron Cohen would take it, they had been dispelled that very morning. Ali G was invited on to Sara Cox's Radio 1 breakfast show, to talk about 'Me Julie', the single he has just recorded with Shaggy which is to be part of the soundtrack to his upcoming movie, Ali G in Da House.Even though the audience couldn't see him, the man sitting in the chair was Ali G, complete with shades; it was, therefore, Ali G who forced Cox to issue heartfelt apologies to her listeners, after he filled the air with expletives. Radio 1 doesn't like people swearing on air before 9am.

According to his press people - and he has many these days - 'Ali G did not apologise' for using the word 'motherfucker'. It would not be in character to do so. Radio 1 agrees that he didn't but claims that Sacha did say sorry to the show's producers. Ali G's spokeswoman will not comment on that. After all she doesn't represent Sacha Baron Cohen. She only represents someone called Ali G.

Keeping himself and his creation separate makes an awful lot of sense for Baron Cohen. It means he never has to engage with the debate over his act. Is Ali G taking the piss out of the macho, misogynist excesses of black youth culture? Is he taking the piss out of Wiggers, white boys who want to be black, or - with the name Ali - is it Asians he's after? There are many who say it doesn't matter; that the only thing which counts is that he's funny. As to Baron Cohen, he never apologises, never explains.

Sacha Baron Cohen was born in 1972, the middle of three sons. His was a standard Orthodox Jewish upbringing. His father, Gerald, is in the schmatte business and owns a menswear shop in Piccadilly. Gerald's Israeli wife, Daniella, is described, rather tautologously, as a 'forceful Jewish mother' who would regularly encourage her talented boys to give musical recitals to guests after Friday night Shabbat meals. Baron Cohen has stayed close to that upbringing. He is an observant Jew who keeps kosher and still regularly goes back to his parents' house on Friday nights, sometimes taking his new friends with him. Not too long ago Madonna, who cast Ali G in the video for her single 'Music', was a Shabbat guest of the Baron Cohens.

Even the choice of school was of a piece: he went to Haberdashers' Aske's, a public school in Elstree which, despite its Anglican roots, has become a firm favourite of the Jewish community for their smart, academically inclined sons. David Baddiel also went there, as did much of the eventual writing and production team behind Ali G. He studied history at Cambridge, where for a while he focused on the American Civil Rights movement, and was regarded as an immensely able student, perhaps with a future in academia.

But performance always attracted him. As a kid he had been a part of Habonim, a Jewish youth movement with a particular fetish for producing sketch shows. 'He was always doing characters, doing performances,' says a contemporary. At Cambridge he was in the Footlights and on graduating he tried his luck on the London comedy circuit. Together with his younger brother, Erran, he ran a comedy club in Hampstead where they performed a song called 'Shvitzing' (Yiddish for sweating), about two old Hassidic Jews who would strip down to their underpants because their black felt garb was too hot. They took the sketch to the BBC. It was turned down for being offensive.

For all his hunger to perform, friends do not remember him as particularly gregarious or outgoing in those days: he was shy, they say, quiet, perhaps even a little geeky. That hasn't changed much even now that his character has found fame. 'If you sat next to him at dinner,' one friend says, 'you might come away thinking how sweet and clever he is, and so very funny for an accountant.'

His big break came in 1998 when comedy producer Harry Thompson was looking for new talent for Channel 4's 11 O'clock Show . Baron Cohen was working for the Paramount comedy cable channel, appearing as Borat, a TV presenter from Kazakhstan wearing the sort of safari suit his dad's shop had been selling for years and asking dumb questions with a straight face. In one clip he asked a country hunt whether prison overcrowding could be dealt with by releasing prisoners and letting the hounds chase them down. The hunt described it as 'a bloody good idea'. The seeds of Ali G's interviews had been sown.

Today there are as many people who claim to have created Ali G as there were in the audience at the Brits last Wednesday night. Certainly Thompson lays claim to be one. 'If he had a whiff of Islam about him, we thought people would be afraid to challenge him,' he says. It worked. Serious establishment figures, desperate not to seem out of touch with 'yoof', attempted to answer his questions. He asked Judge James Pickles if it is 'all right to murder someone if they call your mum a slag?' He asked Tony Benn if 'people strike just 'cos they is lazy and want to chill for a day or so'.

Yet the most sublime moment came when, being dragged away from an environmental protest by a policeman, he asked: 'Is it cos I is black?' The policeman ignored the colour of his skin and told him, politely, that his blackness had nothing to do with it. A catch phrase had been born. Ali G has become a massive, money-spinning cult.

But the early joke was soon blown and over the past few years Ali G has had to be repositioned. If anything, that has made the contradictions more pronounced. A few years ago, figures in the black community were describing him as a deeply offensive stereotype. Last week, by joining up with Shaggy, Baron Cohen started moving his one bankable product into the heart of that culture. Is he now ridiculing its misogyny and the apparent adoration of gun violence, or is he co-opting it? (And what of the deep-seated anti-Semitism within parts of the black community?)

Of course, he will not say. He never does. But nothing he is doing is casual or opportunistic. 'The attention to detail is absolutely staggering,' says one person who has worked with him. Even the BBC will admit, off the record, that they believe Ali G's lurch into profanity on Radio 1 last week 'was premeditated'. His new movie, in which Ali G becomes the MP for Staines, is released next month. There's a lot of investment that needs to be earnt back. He needs to keep the boy's profile high and he worked out how to do it.

That said it is not the first time he has used expletives to get an effect. At the beginning of his 1999 video innit he stares into camera and says: 'To get an 18 [certificate] I is gonna 'ave to use a word which I 'as never used before: c-t.' The video got a 15 certificate. Which, doubtless, is exactly what Sacha Baron Cohen intended.